Physiology Flashcards
what is automaticity
the ability of the heart to beat in the absence of external stimuli
what is normal sinus rhythm
automaticity originating from the sino atrial node
where is the SA node located
within the right atrium near the entrance of the SVC
what causes the wave of depolarisation within the SA node
K+ efflux
what causes the wave of repolarisation within the SA node
activation of L-type calcium channels and influx of calcium
describe calcium induced calcium release
increasing the concentration of intracellular calcium causes more to be released from the SR
what is the spread of the conduction of the heart after the SA node
atrio-ventricular node - bundles of His - perkinje fibres
the inner wall of the ventricles conduction is supplied by
perkinje fibres
where is the AV node located
base of the RA
the AV node slows/speeds up conduction between atria and ventricles
slows down
why is the speed of conduction slowed down at the AV node
to allow the atria to complete systole before the ventricles begin to contract
the AVN is the only point of conductivity between atria and ventricles true/false
true
in ventricular muscle action potential what is the change in voltage between phase 0 and phase 1
-90mV to +20mV
what does phase 0 of ventricular action potential represent
rapid Na influx
what does phase 1 of ventricular action potential represent
closure of Na channels
K efflux
what does phase 2 of ventricular action potential represent
Ca influx through L-type channels
what is phase 2 also known as and why
plateau phase because membrane potential is very close to its peak for some time
what does phase 3 represent
closure of Ca channels
K efflux
what does phase 4 of ventricular action potential represent
resting membrane potential at -90mV
closure of K channels
sympathetic nervous system __ heart rate and conductivity
increases
sympathetic nerves act act on __ mediated by __
B1 adrenoceptors mediated by adrenaline/noradrenaline
parasympathetic nerves ___ heart rate and conductivity
decrease conductivity but no effect on nodal cells
parasympathetic innervation through CN__
X
vagal tone increases/decreases the slope on pacemaker potential
decreases
cardiac muscle is striated/non-striated
striated
what gives cardiac muscle its striated appearance
dark and light bands of myosin and actin
actin slides over myosin or myosin slides over actin?
actin slides over myosin
muscle fibres are arranged into what functional unit
sarcomeres
define stroke volume
volume of blood ejected by each ventricle per heartbeat
EDV - SSV
what does the frank-starling curve show
the more a ventricle is filled during diastole due to stretch of the myocytes, the greater the volume of ejected blood will be during followed contraction
heart failure has a negative inotropic effect - what does this mean
decreases contractility of the heart
define cardiac output
volume of blood ejected by each ventricle per minute
stroke volume x heart rate
what is the cardiac cycle describing
stages occurring from the beginning of one heart beat o the beginning of the next
what are the 5 events in the cardiac cycle
passive filling atrial contraction isovolumetric ventricular contraction ventricular ejection isovolumetric ventricular relaxation
what occurs during passive filling
pressure between atria and ventricles almost zero - AV valves open allowing passive venous return into ventricles
what % of blood moves from atria to ventricles by passive filling
80%
what occurs during atrial contraction
remaining volume moves between atria and ventricles
occurs between P and QRS complex
what happens during isovolumetric ventricular contraction
AV valves shut and pressure within the ventricles rises
what happens during ventricular ejection
ventricular pressure > aortic/pulmonary pressure so AV valves open
what happens during ventricular relaxation
blood moves through AV valves causing closure
ventricular pressure < atrial pressure so meaning AV valves open and new cycle starts
what is blood pressure
the outwards hydrostatic pressure exerted by the blood on vessel walls
systolic BP is during
contraction
diastolic BP is during
relaxation
the sounds heard when auscultating blood pressure are known as
Korotkoff sounds
1 korotkoff sound represents
systolic blood pressure
5/final sound represents
diastolic pressure - silence
what is the formula for calculating mean arterial blood pressure
(2xdiastolic + systolic)/3
1/3(systolic-diastolic + diastolic)
what is the normal range for mean arterial blood pressure
70-105mmHg
what are the main resistance vessels in the body
arterioles
what controls the short term regulation of blood pressure
baroreceptors
where are the baroreceptors located
aortic and carotid arteries
which cranial nerve supplies the carotid baroreceptor
glosspharyngeal nerve (CN IX)
which cranial nerve supplies the aortic baroreceptor
vagus nerve (CN X)
how do the baroreceptors change MAP
pressure sensors detect change in MAP - send signal to medulla - triggers effectors in heart to alter HR, SV, SVR
if blood pressure is high - baroreceptors causes HR, SV, SVR to increase/decrease
all decrease
what is postural hypotension
failure of the baroreceptors to respond immediately to a change in gravitational shift
postural hypotension is defined as
drop in systolic bp by 20mmHg or drop in systolic bp by 10mmHg plus dizziness, lightheadedness or blurred vision
what controls the long term regulation of blood pressure
regulation of extracellular fluid (composed of plasma volume and interstitial fluid) - through excess/deficit water and Na
what 3 hormonal systems regulate bp long term
RAAS
natriuretic peptides
ADH
what substance is released from the kidneys when bp is low
renin
renin goes on to activate __ regulated by which enzymes
angiotensin I - angiotensin II - aldosterone
angiotensin converting enzyme responsible for change
what effect does aldosterone have once released from adrenal cortex
causes sodium and water retention causing blood pressure to rise
natriuretic peptides provide the same/different response as RAAS
different - acts counter regulatory mechanism causing excretion of sodium and water
where is ANP released from
atria due to atrial distension
where is BNP released from
the brain in response to stretch in the ventricles
what is ADH
peptide hormone derived from pre-cursor in the hypothalamus
secretion stimulated by increase in extracellular fluid volume
what is the effect of ADH release
increases water reabsorption in kidney tubules and vasoconstriction of blood vessels
resistance to blood flow is directly proportional to
blood viscosity and length of vessel
resistance to blood flow is inversely proportional to
radius of the blood vessel to the power of 4
what is vasomotor tone
due to sympathetic stimulation blood vessels are partially constricted at rest
do parasympathetic nerves innervate arterial smooth muscle
no - except in the penis and clitoris
name some humoral agents that cause smooth muscle dilatation
histamine
bradykinin
nitric oxide
name some humoral agents that cause smooth muscle constriction
thromboxane
leukotrienes
define the term shock
abnormality of circulatory system resulting in decreased tissue perfusion
what is hypovolaemic shock
shock due to loss of blood volume - can be due to trauma, vomiting, diarrhoea
what causes cardiogenic shock
sustained hypotension due to reduced cardiac contractility. results in hypoperfusion of vessels
pneumothorax is an example of which type of shock
obstructive - SVC becomes obstructed and inter-thoracic pressure increases
what causes vasoactive shock
release of vasocative mediators such as NO causing excessive vasodilation
what causes neuorgenic shock
loss of sympathetic tone causing excessive vasodilation
compensatory mechanisms can maintain BP until how much blood is lost
30% of total volume
what is the bodys response to shock
increase sympathetic tone and vasoconstriction
what are some adaptations of coronary circulation to allow high oxygen demand
high number of capillaries
high basal blood flow (at rest)
high oxygen extraction rates
if the heart wants to increase oxygen to area what is done
must increase coronary blood flow because oxygen rate in blood is already so high
if coronary PO2 is low blood vessels vasodilate/vasoconstrict and why
vasodilate and oxygenation to that are needs to be improved
where specifically in the brain is very sensitive to hypoxia
grey matter
which arteries supply the brain
internal carotids
vertebral arteries
which arteries make up the circle of Willis
basilar and vertebral arteries
what range must cerebral blood pressure be within for correct perfusion and autoregulation
60-100mmHg
if MAP rises in cerebral circulation blood vessels dilate/constrict
dilate to decrease blood pressure
what blood pressure causes fainting and confusion in cerebral circulation
50mmHg or less
decrease in PCO2 causes vessels to
constrict
what causes a rise in intercranial pressure
trauma etc
what does an increase in intercranial pressure do to cerebral perfusion pressure and cerebral blood flow
reduces both
which substances can and cannot penetrate the blood brain barrier
O2 CO2 and glucose can all penetrate BBB
ions and proteins cannot
why can specific substances not pass the BBB
protects the neurones
what is the pressure within the pulmonary circulation
5-8mmHg
what mechanism prevents oedema in pulmonary circulation
absorptive forces are greater than filtration forces
hypoxia causes vaosdilatation/vasoconstriction in pulmonary arteries
vasoconstriction - diverts blood from poorly ventilated area of the lung
during exercise what happens to blood floow of skeletal muscle
increases
in skeletal muscle what overrides sympathetic vasoconstrictor activity
metabolic hyperaemia
